Solar light degradation of organic dye pollutants and preparation of bis(indolyl)methanes using core-shell Fe3O4@SiO2@CuO nanocomposite

In this research, a new ferromagnetic-recoverable core-shell Fe3O4@SiO2@CuO nanocomposite of a certain size (20–25 nm) has been synthesized based on Cu(II) complex coated on Fe3O4@SiO2 nanoparticles by facile and fast solid state microwave irradiation method. The photocatalytic activity of the nanocomposite was investigated for degradation of methylene blue (MB) and methyl orange (MO) dye pollutants in aqueous media under solar light irradiation. The nanocomposite could destroy these dyes with high efficiency in short time. With comparison of degradation percentages can be concluded that the nanocomposite shows better photocatalytic activity for MB dye (97% in 180 s). Kinetic study revealed higher rate constant for degradation of MB (k= 3.6×10−3 s−1) with pseudozero-order model. Also, Fe3O4@SiO2@CuO nanocomposite was an efficient magnetically recoverable catalyst for the preparation of bis(indolyl)methanes (BIMs) through the condensation of an aldehyde with 2 equivalents of indole in EtOH/H2O as green solvents.

Recently, we introduced solid-state microwave method as a newer and 'greener' synthetic methodology for the preparation of NiO nanoparticles [39].This method has interesting features and might find application for the preparation of nanomaterials with improved properties which can provide considerable benefit in the fields of medicine.We also used this method for the preparation of Co-Sn-Cu oxides/graphene nanocomposites as green and recyclable catalysts for preparing 1,8-dioxo-octahydroxanthenes and apoptosis-inducing agents in MCF-7 human breast cancer [40].The superiority of solid-state microwave method over the above mentioned conventional methods [8,41,42] for the synthesis of core-shell systems are reduction of physical and chemical cost, simplicity, safety, generation of pure nanoparticles as well as this method is green (no surfactant) and fast.Along this line, herein, we want to synthesize core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite by solid-state microwave method based on a novel nano-sized Fe 3 O 4 @SiO 2 @CuL precursor, prepared from the reaction of the Fe 3 O 4 @SiO 2 with a copper (II) Schiff base complex (CuL).Photocatalytic activity of the synthesized core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite was investigated for degradation of the cationic and anionic organic dye pollutants (MB and MO, respectively) under solar light.Also, this nanocomposite was applied as a new heterogeneous catalyst for the preparation of BIMs under mild conditions.

Experimental 2.1. General
The reagents and solvents were either prepared in our laboratory or were purchased from commercial suppliers Merck and Fluka.For the microwave irradiation, a microwave oven (LG: MH6535GISW, 1700 W, Korea) was used.Ultrasonic (US) generator was carried out on ultrasonic probe (Top-Sonics UPH-400, Germany).Proton nuclear magnetic resonance ( 1 H NMR) spectra were recorded on 400 MHz spectrometer (Bruker, Germany) in deuterated chloroform.Fouriertransformed infrared spectroscopy (FT-IR) was used to obtain spectra of samples using a Schimadzu system FT-IR 8400 spectrophotometer (Japanese) by KBr pellets.X-ray diffraction (XRD) analysis was carried out using a Rigaku D-max C III, X-ray diffractometer with Ni-filtered Cu Ka radiation (PANalytical X'Pert Pro, Netherlands).The morphology of samples was founded with field emission scanning electron microscopy (FESEM) that was taken on a Hitachi s4160/Japan with gold coating that was equipped with a link EDX analyzer.The magnetic property of the sample was measured by a vibrating sample magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran) at room temperature.UV-Vis spectra were measured on a double-beam Shimadzu 1650 PC UV-Vis (Japanese) and the samples were dispersed in 20 mL EtOH at room temperature for 20 min.Melting points were found out using a Fisher-Jones melting-point apparatus (USA).
Bis Gd 3+ complex, Na salt Scheme 1. Biological applications of a series of BIM derivatives.

Preparation of the nano-sized Fe 3 O 4 @SiO 2
Fe 3 O 4 magnetic nanoparticles were prepared by Fathirad's method [43].Firstly, a solution of 20 mL FeSO 4 .2H 2 O (1.6 g, aq.) and 50 mL FeCl 3 .6H 2 O (3.8 g, aq.) was prepared.Then, 10 mL NH 3 solution (25%) was slowly added to the above solution at the presence of ultrasonic irradiation (100 W) for 15 min under N 2 atmosphere.After a few minutes, a black suspension was generated and then the Fe 3 O 4 magnetic nanoparticles were separated using an external permanent magnet.The nanoparticles were washed with distilled water, ethanol, and dried under vacuum at 70 °C.After the preparation of Fe 3 O 4 nanoparticles, Fe 3 O 4 @SiO 2 structure was prepared by coating the Fe 3 O 4 nanoparticles with tetraethyl orthosilicate (TEOS).For this purpose, to 30 mL ethanolic suspension of the obtained Fe 3 O 4 was slowly added 4 mL TEOS.Then, 12 mL aqueous solution of ammonia (25 %) was added to this solution in the presence of ultrasonic irradiation (100 W) for 10 min.The dark brown precipitate, Fe 3 O 4 @SiO 2 , was formed with stirring at room temperature after 24 h.The obtained Fe 3 O 4 @SiO 2 was filtered off, washed with methanol, and dried under vacuum at room temperature.

Preparation of copper (II) Schiff base complex (CuL)
H 2 L ligand (0.3 mmol, 0.15 g), in methanol (10 mL), was exposed to ultrasonic irradiation (150 W).Afterwards, 10 mL methanolic solution of Cu(CH 3 COO) 2 .2H 2 O (0.3 mmol, 0.14 g) was added dropwise to the above mixture, followed by 15 min sonication at room temperature.The mixture was filtered and the CuL powder (brown color) was washed successively with methanol and diethyl ether, and dried in air at ambient temperature overnight [45].2.5.Preparation of the nano-sized Fe 3 O 4 @SiO 2 @CuL precursor A suspension of Fe 3 O 4 @SiO 2 (1 g) in chloroform (50 mL) was prepared by sonication and then excess amount of CuL Schiff base complex (1.2 g, 2 mmol) was added dropwise to the prepared suspension under ultrasonic irradiation for 30 min.The obtained dark brown suspension was stirred at room temperature for 12 h, afterwards the Fe 3 O 4 @SiO 2 @CuL product was filtered off, washed twice with chloroform and diethyl ether, and dried under vacuum at room temperature overnight.2.6.Preparation of the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite To prepare core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite, 2 g of the Fe 3 O 4 @SiO 2 @CuL precursor was poured into a porcelain crucible and it was placed in another bigger porcelain crucible, filled with CuO powder (as microwave irradiation absorber).The collection was placed in a microwave oven under microwaves irradiation in air (950 W, 350 °C).The generated heat led to the decomposition of precursor sample.After 10 min, decomposition of the Fe 3 O 4 @SiO 2 @ CuL precursor was completed.The Fe 3 O 4 @SiO 2 @CuO nanocomposite product was washed with ethanol and dried under vacuum at room temperature overnight.

Photocatalytic tests
The photocatalytic ability of the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite was surveyed for the removal of MB or MO as organic dye pollutants from aqueous solutions.Firstly, the dosage of photocatalyst (0.004-0.007 mg), amount of H 2 O 2 (0-5 mL), and pH of the solution (5-10) were selected and optimized.The photocatalytic tests were performed on the days (10 AM to 2 PM) of bright sunny light (average light intensity of 180 mW cm -2 ).Typically, 0.005 g of the Fe 3 O 4 @SiO 2 @CuO nanocomposite as photocatalyst was added to 50 mL of MB or MO aqueous solution with concentration of 4 ppm and the mixture was stirred (500 rpm) at room temperature in dark for 30 min, in order to establish an adsorption-desorption equilibrium between catalyst and dye.Then, the mixture was exposed to solar light in the presence of an appropriate amount of H 2 O 2 (30%) at suitable pH, and consequently the degradation process of the dye took place.The degradation of MB and MO dyes was carried out at pH = 7 using 2 mL H 2 O 2 and at pH = 9 using 1 mL H 2 O 2 , respectively.At determined time intervals, the photocatalyst powder was isolated by centrifugation of 3 mL of the mixture and the degradation process of each dye was estimated by measuring the absorptions of MB and MO dyes at 663 and 462 nm, respectively, on their corresponding UV-Vis spectra.The pH of solution was adjusted to determined values by the dropwise addition of HCl (1 M) or NaOH (1 M) to the solution.The percent of degradation was measured by the following equation (Eq.( 1)): , where C t is the final concentration of dye solution after a determined time (t) and C o is the initial concentration of dye solution.After each experiment, the Fe 3 O 4 @SiO 2 @CuO nanocomposite was separated from the solutions by an external permanent magnet and then it was washed several times with water and ethanol and dried at 70 °C, and reused for the next experiments.

Typical procedure for the preparation of BIMs
A mixture of 2-thienyl carbaldehyde (1 mmol) and indole (2 mmol) was placed in a round-bottom flask containing 5 mL of EtOH/H 2 O (1:1).Subsequently, Fe 3 O 4 @SiO 2 @CuO nanocomposite catalyst (0.03 g) was added to the mixture and stirred at 80 °C.After the completion of the reaction as followed by TLC (chloroform), the catalyst was separated by an external permanent magnet.The remaining mixture was concentrated on a rotary evaporator under reduced pressure to give the desired product.The products were purified by recrystallization from ethanol or chromatographed on silica plates with chloroform as eluent where necessary.

Synthesis and structural characterization of nanocomposites
The core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite was prepared from Fe 3 O 4 @SiO 2 @CuL as a new precursor by solidstate microwave method (Scheme 2).In the first step, Fe 3 O 4 nanoparticles reacted with TEOS to afford the core-shell Fe 3 O 4 @SiO 2 [43].In the second step, the nano-sized new copper (II) Schiff base complex (CuL) was synthesized from the chemical reaction of 1,2-diaminocyclohexane with 5-bromosalicylaldehyde, followed by the reaction with copper salt, Cu(CH 3 COO) 2 .2H 2 O, under ultrasonic irradiation [44,45].Afterwards, Fe 3 O 4 @SiO 2 @CuL precursor was prepared from the treatment of Fe 3 O 4 @SiO 2 with CuL Schiff base complex under ultrasonic irradiation.In the next step, Fe 3 O 4 @ SiO 2 @CuL precursor was exposed to microwave irradiation in order to be converted to core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite.However, the precursor remained unchanged for 30 min, which shows the compound cannot absorb microwaves.Therefore, it seems that attendance of microwave absorber is required.For this purpose, CuO powder was used as microwave irradiation absorber.By using CuO, we observed that the precursor was completely decomposed during the absorption of heat from the hot CuO and the nano-sized Fe 3 O 4 @SiO 2 @CuO powder was generated through the equation below (Eq.( 2)) [46].Note that in the synthesized core-shell Fe 3 O 4 @SiO 2 @CuO, Fe 3 O 4 is a magnetic core which increases magnetic property of particles and SiO 2 is chosen as an intermediary layer for the connection between shell layer (CuO) and the core (Fe 3 O 4 ).
Eq. ( 2) Figures 1a and 1b show the FT-IR spectra of the nano-sized CuL complex and Fe 3 O 4 @SiO 2 @CuL precursor, respectively.In Figure 1b, the band at 1609 cm -1 was attributed to C=N group that show the red shift to lower frequency compared with those of CuL complex (1640 cm -1 ) [45] as a result of the metal complex formation.The peaks at 1095 and 466 cm -1 were assigned to Si-O and Fe-O bands, respectively [47].By comparison of the IR spectrum of the nano-sized Fe 3 O 4 @ SiO 2 @CuO (Figure 1c) with Fe 3 O 4 @SiO 2 @CuL precursor (Figure 1b), it will be clear that the bands of CuL complex were diminished and stretching frequencies of Si-O and Fe-O bands shifted toward 1097 and 467 cm -1 , respectively.In addition, in Figure 1c, the distinct band at 542 cm -1 was related to the Cu-O band in monoclinic phase [48] and the bands at 1630 and 3300-3400 cm -1 could be assigned to the bending vibration and stretching vibration of H 2 O absorbed by KBr pellets or the sample, respectively [45].Thus, the IR spectral results indicate the successful decomposition of Fe 3 O 4 @SiO 2 @CuL precursor by solid-state microwave method and formation of Fe 3 O 4 @SiO 2 @CuO nanocomposite.Figure 2 exhibits XRD analysis of the synthesized Fe 3 O 4 @SiO 2 @CuO nanocomposite.The XRD result manifests the peaks of CuO in monoclinic phase (Space group: Cc, No: 9).The crystallographic parameters of a, b, and c are 4.69, 3.42  and 5.13 Å, respectively.Also, values of α, β, and γ parameters are 90.00°,99.54°, and 90.00°, respectively.The significant peaks appeared at 2θ = 35.77°,38.95°, and 49.07° that can be exactly related to (-1 1 1), (1 1 1), and (-2 0 2) planes of crystal, respectively.In this pattern, only CuO (JCPDS Card No. 80-1916) and Fe 3 O 4 (JCPDS Card No. 75-0449) phases were observed.It is obvious that SiO 2 have amorphous structure [49].As shown in Figure 2, there are no peaks of impurity, suggesting that the pure crystalline Fe 3 O 4 @SiO 2 @CuO was formed via solid state decomposition of Fe 3 O 4 @SiO 2 @CuL precursor under microwave irradiation.Also, in the XRD pattern, wide width of the peaks is due to the formation of small size particles of the Fe 3 O 4 @SiO 2 @CuO (Table 1).The mean size of the Fe 3 O 4 @SiO 2 @CuO particles calculated by the Debye-Scherrer equation was found to be 23.83 nm [50].
The morphology of the synthesized Fe 3 O 4 @SiO 2 @CuL precursor and Fe 3 O 4 @SiO 2 @CuO nanocomposite were investigated by FESEM analysis.The images of the Fe 3 O 4 @SiO 2 @CuL precursor were shown in Figures 3a and 3b at different magnification.As it can be seen, the morphology of the precursor is nanorod.However, from Figures 3c and 3d, it is clear that the morphology of Fe 3 O 4 @SiO 2 @CuO particles are spherical shape and they are quite different from that of   the precursor compound.Therefore, these results reveal that Fe 3 O 4 @SiO 2 @CuL precursor was converted to Fe 3 O 4 @SiO 2 @ CuO particles by solid-state microwave method.Figure 4a displays TEM analysis of the Fe 3 O 4 @SiO 2 @CuO nanocomposite.A typical image of the synthesized sample shows core-shell shape of uniform nano crystalline structures.The black spot shows Fe 3 O 4 core which is surrounded by SiO 2 .Also, ashen parts after SiO 2 regions illustrate CuO shell.The particle size distribution of nanocomposite is 20-25 nm (Figure 4b).The particle size estimated by XRD diffraction pattern and the TEM analysis have good match with each other.
EDX spectrum of the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite is illustrated in Figure 5 and contains signals of Cu, Fe, Si, and, O elements.The Au and Si signals (notated as coating) were observed due to the instrument.Additionally, the weight and atomic percentages of the resided elements in obtained nanocomposite have been shown in a table inserted in Figure 5. Therefore, from the above results, it can again be concluded that Fe 3 O 4 @SiO 2 @CuO nanocomposite was papered by solid-state microwaved method.
The alteration in magnetization (M) vs. applied field (H) for the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite at room temperature with field sweeping from −15,000 to +15,000 Oe is shown in Figure 6.The hysteresis loop shows a weak ferromagnetic behavior.The hysteresis loop of nano-sized materials was related to the magnetic anisotropy of the lattice, domain structure (pinning effect of magnetic domain walls at grain boundaries), as well as impurities within the nanosized structures [51].The remnant magnetization (Mr) and saturation magnetization (Ms) were found to be 0.25 and 3.22 emu g -1 , respectively.The value of coercive field (Hc) was estimated as 0.053 Oe.The magnetic property of synthesized core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite may be attributed to the different parameters such as sample shape, size, crystallinity, magnetization direction, and synthetic method.
The optical property of the prepared Schiff base complex CuL, nano-sized Fe 3 O 4 @SiO 2 @CuL precursor, and core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite was studied by UV-Vis spectroscopy.Figure 7a indicates the absorbance spectrum of the Schiff base complex CuL.The bands at 200-300 nm were due to π-π* and n-π* transitions.The band at around 380 nm was related to 2 B1g → 2 Eg transition at D 4 h field [52].In the Fe 3 O 4 @SiO 2 @CuL spectrum (Figure 7b), the strong absorption band was observed at around 250 nm, which is due to the π-π* transitions of the phenolic rings [53].Also, the bands  appeared at 300-400 nm were related to the charge transfer transitions, MLCT and LMCT (metal ligand charge transfer and ligand metal charge transfer, respectively), which have shifted in comparison with those of CuL complex [45], due to the formation of Fe 3 O 4 @SiO 2 @CuL.In Figure 7c, the bands observed are related to the electronic transition from Cu (3d) to O (2p) orbitals [54].
The band gap of the semiconductors can be calculated by Tauc's equation (Eq.( 3)): (αhυ) 1/n = A(hυ-E g ), where α, hυ, A, and E g are coefficient of absorption (cm -1 ), energy of photon (eV), proportionality constant, and the band gap energy (eV), respectively.The value of the exponent denotes the nature of the electronic transition (allowed or forbidden), and whether it is direct or indirect: n =1/2 for direct allowed transitions, n = 3/2 for direct forbidden transitions, n = 2 for indirect allowed transitions, and n = 3 for indirect forbidden transitions.Plotting the (αhυ) 1/n versus (hυ) is a matter of testing n =1/2 or n = 2 to compare which provides the better fit and thus identifies the correct transition type.Figures 7d and 7e show curves of (ahυ) 2 -hυ for the Fe 3 O 4 @SiO 2 @CuL precursor and core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite, respectively, showing a direct allowed transition.The linear region has been used to extrapolate to the X-axis intercept to find the E g value.Using this concept, the E g values of Fe 3 O 4 @SiO 2 @CuL precursor and core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite were found to be 4.7 and 3.2 eV, respectively.E g values of Fe 3 O 4 and Fe 3 O 4 @SiO 2 have been found to be 1.3 and 1.68 eV, respectively [55].As it can be seen, E g of core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite has shifted in comparison with those of Fe 3 O 4 @SiO 2 @ CuL precursor.The band of core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite shows red shift toward CuO thin films [56] or blue shift in comparison with quantum dots of CuO [57].This difference is probably related to morphology, size, and effect of the present elements or synthetic method.
Because of a moderate band gap (3.2 eV) of core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite, we anticipated that it can act as a photocatalyst for destroying the dye pollutants.For this purpose, the photocatalytic activity of core-shell Fe 3 O 4 @ SiO 2 @CuO nanocomposite was investigated for solar light degradation of MB and MO organic dye pollutants.Firstly, the experiments were carried out for degradation of MB as a typical cationic dye.The degradation of MB dye was investigated at 663 nm at which the dye shows a strong absorption.Figure 8a shows the values of C t /C 0 of MB dye in different conditions.Clearly, the C t /C 0 values under the optimized conditions (pH= 7 and 2 mL of H 2 O 2 ) strongly decreased at room temperature duo to the solar light degradation process.The H 2 O 2 acts as an assisted-degradation and produces more free radicals (HO•), which leads to faster and more effective degradation.However, use of further amounts of hydrogen peroxide, up to critical concentration, will not enhance the rate of dye degradation process [58].
The characteristic absorption bonds of MB dye at optimized conditions at 663 nm at different times are given in Figure 8b.Clearly, the characteristic absorption decreases with the passage of time.The absorption of MB is about zero after 180 s solar light irradiation.During the degradation process, the intense blue color of the initial solution was decreased until it becomes almost colorless, indicating the successful solar light degradation process of MB.It is worth noting that the absorption bands of MB were not shifted at 663 nm, denoting that the degradation of MB is due to the degradation of the chromophore groups [59].Thus, the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite is an efficacious photocatalyst for the degradation of MB dye in short time with a degradation efficiency of 97 %.In order to demonstrate the effectiveness of photons in dye degradation processes, typically, 0.005 g of the Fe 3 O 4 @SiO 2 @CuO nanocomposite was added to 50 mL of MB or MO aqueous solution with concentration of 4 ppm and the mixture was stirred (500 rpm) at room temperature in dark for 30 min.UV-Vis spectra showed that the significant absorption peaks of dyes were observed without decreasing.Therefore, dye degradation is accomplished in the presence of light.Based on this encouraging result, the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite was subsequently extended to the degradation of MO as a typical anionic dye in similar conditions.The experimental tests showed that the values of C t /C 0 of MO dye under optimized conditions (pH = 9 and 1 mL of H 2 O 2 ) decreased sharply at room temperature (Figure 8c).The intensity change of the absorption bands at 462 nm over of the Fe 3 O 4 @SiO 2 @CuO nanocomposite catalyst is plotted in Figure 8d as a function of time.After 25 min, the degradation efficiency for MO was estimated as 72 %.When the results obtained for the degradation of MB and MO dyes using the Fe 3 O 4 @SiO 2 @CuO nanocomposite were compared with each other, it was revealed that the degradation of MB dye (the cationic dye) was performed with higher yield in a shorter time, which indicates that the surface of the Fe 3 O 4 @SiO 2 @CuO nanocomposite catalyst is presumably negatively charged.
A radical mechanism for the solar light degradation of MB or MO dyes over core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite is proposed in Figure 9 [55].During irradiation, Fe 3 O 4 @SiO 2 @CuO nanocomposite (E g = 3.2 eV) can create electron-hole pairs (Eq.( 4)).The electrons on the conductive band (CB) react with H 2 O 2 molecules to produce HO• and •O 2 -radicals (Eqs.5 and 6).Meanwhile, holes on the valence band (VB) would be reacted with the H 2 O molecules or OH − ions to form HO• radicals (Eq.( 7)).The produced active HO• and •O 2 -radicals effectively degrade the dye molecules to CO 2 , H 2 O and other inorganic products (Eqs.( 8) and ( 9)).Fe3O4@SiO2@CuO nanocomposite (0.005 g) as a photocatalyst.The solar light degradation kinetics of MB (Figure 10a) and MO (Figure 10b) dyes were determined.The pseudo-zero order model was used: C t = -k t + C 0 , where C 0 and C t are the dye concentrations before and after solar light irradiation, respectively, t is the reaction time as well as k is the rate constant.As shown in Figure 10, the k values for the degradation of MB and MO dyes were found to be 3.6 × 10 -3 s -1 and 0.213 × 10 -3 s -1 , respectively.The results indicate that the MB degradation rate is more than those of MO dye.The k value for the photodegradation of MB dye using the Fe 3 O 4 nanoparticles has been found to be 1.7 × 10 -4 s -1 [60], lower than that of Fe 3 O 4 @SiO 2 @CuO nanocomposite (3.6 × 10 -3 s -1 ), thus, the latter ones seem to be more effective for dye degradation process.
One of the important issues in the catalytic experiments is stability and reusability of the catalysts.The synthesized core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite is a magnetic material and after each reaction it was easily segregated using an external permanent magnet.After washing core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite severally with deionized water, the solid was dried and reused for consecutive cycles for the degradation of MB or MB dyes.The results showed that Fe 3 O 4 @SiO 2 @CuO nanocomposite can be reused up to five cycles with no significant decline in degradation efficiency (Figure 11a).Negligible loss in its activity is due to loss of catalyst during separation or through washing cycles.These results were authenticated with FESEM and EDX spectra of Fe 3 O 4 @SiO 2 @CuO nanocomposite after fifth reuse (Figures 11b and 11c), indicating that the structure of the catalyst was preserved after recovery.Finally, the photocatalytic performance of the synthesized core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite, for degradation of MB and MO dye pollutants was compared with previously reported literature (Table 2).The results showed that Fe 3 O 4 @ SiO 2 @CuO nanocomposite provides better catalytic performances than the previously reported photocatalysts from view of the degradation efficiency and reaction rate.Notably, whereas many catalysts used for the degradation of dyes (entries 1-6) require an additional energy (UV irradiation, ultrasound or microwave source) or heating conditions, Fe 3 O 4 @SiO 2 @   CuO nanocomposite does not need an external energy and the degradation process of dyes was carried out only under solar energy.Noteworthy is that in a recent work [66], nanoplate of mixed-Fe 3 O 4 @SiO 2 @CuO synthesized by precipitation method was reported as a photocatalyst for destroying the MO dye.However, the degradation efficiency was low (17.6%) and the degradation process could be accomplished only under UV-Vis irradiation.The superiority of our core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite over nanoplate of mixed-Fe 3 O 4 @SiO 2 @CuO seems to depend on the differences in morphology and type of precursor, and also applied synthetic method.In the solid-state microwave decomposition method, at the molecular level, microwaves interact with the reactants and the electromagnetic energy is generated.This energy change to heat by rapid kinetics of the molecules and can improve the chemical reaction [39].
Encouraged by the results obtained from the photocatalytic efficiency of Fe 3 O 4 @SiO 2 @CuO in degradation of organic dye pollutants, we turned our attention to examine the catalytic activity of this nanocomposite for the preparation of

°C
BIMs.Catalytic experiments were initiated with the reaction of 2-thienyl carbaldehyde (1 equiv.)with indole (2 equiv.)as a typical reaction.To find the optimum reaction conditions, the influence of the solvent nature, reaction temperature, and amount of catalyst were investigated.The best yields were obtained in EtOH/H 2 O (1:1) at 80 °C in the presence of 0.03 g of Fe 3 O 4 @SiO 2 @CuO nanocomposite.After this, the performance of this approach was explored for the preparation of a wide variety of BIMs.BIMs were obtained in excellent yields using Fe 3 O 4 @SiO 2 @CuO nanocomposite from various aliphatic and aromatic aldehydes, having different substituents, and indole (Table 3, entries 1-7, 10-12).Also, heteroaromatic aldehydes smoothly reacted with indole using Fe 3 O 4 @SiO 2 @CuO nanocomposite to yield their corresponding BIMs (entries 8,9).Reaction of ketones with indole was very slow (entry 13) due to steric effects, and the product was afforded only in trace amounts even after extended time (120 min).In all cases, the reaction proceeds smoothly without the formation of any undesirable products, which normally are observed under the influence of strong acid catalysts.The work-up is reduced to a mere separation of the magnetic catalyst and evaporation of the solvent.Figure 12 shows the 1 H NMR spectrum of the 3,3'-(2-thienylmethylene)bis-1H-indole (as a brick red powder).
To explore the actual role of Fe 3 O 4 @SiO 2 @CuO nanocomposite in the synthesis of BIMs, we explain a plausible mechanism of the reaction in Scheme 3.An activated aldehyde carry out an electrophilic substitution reaction at C-3 of an indole, which after loss of water yields intermediate I. Addition of another molecule of indole to intermediate I, like the Michael addition fashion, yields intermediate II, which produces the target product after aromatization takes place via deprotonation.
The reusability of the Fe 3 O 4 @SiO 2 @CuO nanocomposite catalyst in the synthesis of BIMs was studied.The catalyst was reused up to five times without remarkable loss of its efficiency (Figure 13).
A comparison of the present procedure, using Fe 3 O 4 @SiO 2 @CuO nanocomposite, with selected previously reported catalysts is presented in Table 4. Clearly, Fe 3 O 4 @SiO 2 @CuO nanocomposite in addition to having the advantages such as easy separation and recyclability has fine catalytic performance compared to other reported protocols.

Conclusion
In summary, well-defined core-shell Fe 3 O 4 @SiO 2 @CuO composite was prepared via a fast and efficient solid state microwave irradiation.The composite is a ferromagnetic material in the nano scale range of size (20-25 nm) with a moderate band gap     of 3.2 eV, which makes it suitable for applications in areas such as electronics and photonics.The MB and MO dyes were degraded over Fe 3 O 4 @SiO 2 @CuO nanocomposite as a reusable photocatalyst with 97% and 72% efficiency, respectively.The important feature of the present protocol was the use of solar energy to accomplish the degradation of dye pollutants.Also, the nanocomposite exhibited high catalytic activity in the BIMs synthesis.The catalyst is easily separable by an external magnet and its catalytic activity remains after several reaction cycles.The cleaner reaction profiles, simple workup, no competitive side reactions, high reaction rates, and high yields of the desired products are other advantages of this method.The synthesis and applications of other core-shell nanocomposites with different magnetic cores using solid state microwave method is under investigation.

Figure 6 .
Figure 6.Magnetization versus applied magnetic field for Fe 3 O 4 @SiO 2 @CuO nanocomposite at room temperature (a) and enlarged view of the hysteresis loop in the low-field region (b).

Figure 8 .
Figure 8. Concentration changes (C t /C 0 ) versus irradiation time and time-dependent absorption spectrum during degradation process of a 4 ppm aqueous solution of MB (a,b, respectively) and MO (c,d, respectively) dyes in the presence of the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite (0.005 g) as a photocatalyst.

Figure 9 .
Figure 9.A proposed mechanism of solar light degradation of MB or MO dyes in aqueous solution over the core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite/ H 2 O 2 system with an energy level diagram.CB and VB are the conduction and the valence bands, respectively.

Figure 10 .
Figure 10.Plot of C t versus irradiation time for MB (a) and MO (b) dyes over Fe 3 O 4 @SiO 2 @CuO nanocomposite.The removal of dyes by this nanocomposite followed pseudo-zero order kinetics.

Figure 11 .
Figure 11.Reusability of core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite (0.005 g) in the degradation of a 4 ppm aqueous solution of MB and MO dyes after 3 and 25 min, respectively, (a), its FESEM image (b) and EDX spectrum (c) after fifth reuse.

a
Isolated yield.All products are known compounds and were identified by comparison of their physical and spectral data with those of the authentic samples.

Scheme 3 .
Scheme 3. A probable mechanism of the reaction of aldehydes with indole in the presence of Fe 3 O 4 @SiO 2 @CuO nanocomposite as catalyst.

Table 2 .
Comparing the photocatalytic efficiency of core-shell Fe 3 O 4 @SiO 2 @CuO nanocomposite with some previously reports in degradation of organic dye pollutants.
a Microwave, b reduced graphene oxide, c ultrasound.

Table 4 .
Comparison of the efficiencies of Fe 3 O 4 @SiO 2 @CuO nanocomposite with other reported catalysts for the condensation of indole with benzaldehyde.
a Isolated yields.